When a defect is present in a mask blank, it becomes difficult to accurately transfer a mask pattern formed on a photomask to a semiconductor substrate. Accordingly, it is important to carry out a defect inspection of a mask blank in advance before manufacturing a photomask.
As a method of carrying out a defect inspection of a mask blank, a method of acquiring a dark-field image by using a dark-field optical system is known. When a defect is present in a mask blank, intense scattered light is created from the defective part. Accordingly, in a dark-field image, a defect is observed as a bright point. Therefore, in order to detect a defect signal, a threshold is set in advance, and a signal of a level higher than the threshold is judged to be a defect signal.
However, when the position of the mask blank deviates from the focus position, an image of the defect is not normally formed. For this reason, a problem that the intensity of the defect signal lowers, and the defect cannot be detected is caused. In order to detect the focus position, although it is sufficient if a pattern for detecting the focus position is provided, it is not possible to form a pattern on the mask blank in advance.
As described above, heretofore, it has been difficult to accurately detect a focus position on a substrate surface of a mask blank or the like.
Further, when the position (position based on, for example, a rotational angle of a TDI camera) of an optical system constituent component used for a defect inspection of a mask blank is not appropriate, it is not possible to carry out an inspection of a high degree of accuracy. Accordingly, it is also important to accurately detect the optimum position of the optical system constituent component.
As described above, a method by which it is made possible to accurately detect an optimum imaging position such as an optimum focus position or an optimum position of an optical system constituent component is desired.